clash-of-styles-double-dispatch

DoubleDispatch.cs

using System;

namespace DoubleDispatch
{
    class Program
    {
        static void Main(string[] args)
        {
            var expression = new Addition(
                new MyInt(3),
                new Addition(new MyRational(2, 3), new MyInt(5)));

            var asString = expression.Stringify();
            var result = expression.Eval();

            Console.WriteLine($"{asString} = {result.Stringify()}"); // 3 + 2/3 + 5 = 26/3
        }
    }

    public interface IExpression
    {
        IValue Eval();
        string Stringify();
    }

    public interface IValue : IExpression
    {
        IValue AddValue(IValue operand);
        IValue AddInt(MyInt operand);
        IValue AddRational(MyRational operand);
    }

    public class MyInt : IValue
    {
        public int Val { get; }

        public MyInt(int val)
        {
            Val = val;
        }

        public IValue Eval() => this;

        public string Stringify() => Val.ToString();

        public IValue AddValue(IValue operand) => operand.AddInt(this);

        public IValue AddInt(MyInt operand) => new MyInt(Val + operand.Val);

        public IValue AddRational(MyRational operand)
            => new MyRational(operand.Numerator + operand.Denominator * Val, operand.Denominator);
    }
    
    public class MyRational : IValue
    {
        public int Numerator { get; }
        public int Denominator { get; }

        public MyRational(int numerator, int denominator)
        {
            Numerator = numerator;
            Denominator = denominator;
        }

        public IValue Eval() => this;

        public string Stringify() => $"{Numerator}/{Denominator}";

        public IValue AddValue(IValue operand)
            => operand.AddRational(this);

        public IValue AddInt(MyInt operand)
            => operand.AddRational(this);

        public IValue AddRational(MyRational operand)
        {
            return new MyRational(
                Numerator * operand.Denominator + operand.Numerator * Denominator,
                Denominator * operand.Denominator);
        }
    }

    public class Addition : IExpression
    {
        private readonly IExpression _operand1;
        private readonly IExpression _operand2;

        public Addition(IExpression operand1, IExpression operand2)
        {
            _operand1 = operand1;
            _operand2 = operand2;
        }

        public IValue Eval() => _operand1.Eval().AddValue(_operand2.Eval());

        public string Stringify() => $"{_operand1.Stringify()} + {_operand2.Stringify()}";
    }
}